fix/nested shard reads

[d-v-b]

Fixes #3652 by properly allowing byte range requests in the partial decoding pathway of the sharding codec.

The failure was caused by a certain store-like class (defined just for sharding) failing to handle byte range requests.

There are 2 ways to fix this. One leaves a lot of the cruft in the sharding codec intact and just adds modifies some existing methods. But the other approach removes cruft, and re-uses stuff we have already defined elsewhere in the codebase: the Store API. That's what this PR does. Edit -- this PR now keeps the cruft, with the goal of keeping things small and simple. I just add byte-range support to the shard-specific bytegetter.

In a later PR:

ShardingByteGetter and ShardingByteSetter can be removed, and instead the sharding codec creates Store classes as part of its decoding process. When reading a full shard, a _ShardReaderStore is created, which maps string keys (the names of chunks) onto contiguous ranges in a buffer (the shard bytes). When reading a partial shard, the separate chunk byte regions are used to create a MemoryStore.

Using the Store API here means nested sharding just works, and we get an easy way to add things like caching later.

[codspeed-hq]

Merging this PR will not alter performance

✅ 30 untouched benchmarks

---

_{Comparing d-v-b:fix/nested-shard-reads (bef0907) with main (68359bc)¹}

Footnotes

1. No successful run was found on main (3fa19d2) during the generation of this report, so 68359bc was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[d-v-b]

by making the shape of the array irregular w.r.t to the chunk shape, we hit the partial decode path, which required for evoking the bug reported in #3652

[d-v-b]

setting compressors to None here is also required to trigger the partial decode path.

[codecov]

Codecov Report

❌ Patch coverage is 34.61538% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.55%. Comparing base (7eba7f1) to head (e6784d7).

Files with missing lines	Patch %	Lines
src/zarr/codecs/sharding.py	34.61%	51 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3655      +/-   ##
==========================================
- Coverage   60.88%   60.55%   -0.34%     
==========================================
  Files          86       86              
  Lines       10182    10226      +44     
==========================================
- Hits         6199     6192       -7     
- Misses       3983     4034      +51     

Files with missing lines	Coverage Δ
src/zarr/codecs/sharding.py	51.67% <34.61%> (-10.44%)	⬇️

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